Thin layer chromatography (TLC) is an important chemical analysis method. A thin layer of chromatographic stationary phase, such as silica gel granules and a binder or ion exchange resin granules and a binder, is coated on a suitable panel, such as a pane of glass or a sheet of plastic, to form a TLC plate. A sample is deposited near one edge of the plate and then this edge is immersed into a trough containing a developing solution. The developing solution advances up the TLC plate by capillary attraction in the layer of stationary phase. The components of the deposited sample are chromatographed into separate spots by the advancing developing solution in the layer of stationary phase. These spots are often not visible. Therefore, the TLC plate is usually then sprayed with a chemical that reacts with the components of the spots to form a colored species and thereby reveal the spots. The position of a spot on a TLC plate can be used to help identify the component of the sample responsible for the spot. The size of the spot and the intensity of its color can be used as an indication of the concentration of the component responsible for the spot. One of the primary benefits of the TLC method is that it can be used in situations where the cost and maintenance of complex instrumental methods of chemical analysis would be impractical.
The need for routine inorganic cation determinations is prevalent in several areas of chemical analysis such as water analysis and clinical diagnostics. Specifically, the quantitation of sodium I (Na.sup.+), potassium I (K.sup.+), magnesium II (Mg.sup.+2) and calcium II (Ca.sup.+2) is often required. Common methods for the determination of such cations includes flame photometry, ion selective electrodes, ion chromatography and indirect photometric chromatography.
Indirect photometric chromatography (IPC), as disclosed by Small and Miller in United States Pat. 4,414,842, was an important advance in the art of chromatography. One means of IPC of cations disclosed by Small and Miller was to inject a sample containing a cation of interest, such as sodium I, onto a cation exchange column and chromatograph it through the column with an eluent containing a dilute solution of copper II. The effluent from the column was then passed through a photometric detector set to detect the copper II but not the cation of interest. When the cation of interest passed from the column there was a transient decrease in the concentration of copper II which was seen by the detector as a negative chromatographic peak. It is known that copper II forms an intensely colored complex with ammonia and with diethyldithiocarbamate.
Although IPC is reliable and sensitive, it is not applicable in situations where the cost and maintenance of the required instrumentation is prohibitive. It would be an advance in the art of chemical analysis of cations if an indirect detection thin layer chromatography method could be developed for cations because thin layer chromatography is inherently a less complex method of analysis.